1. Field of the Invention
This invention relates to a system and method for measuring selected parameters of electrical signals having a frequency within a predetermined band of frequencies wherein the electrical signals are derived by relative movement between magnetic transducers and a magnetic medium having prerecorded data thereon and more specifically relates to a system and method for measuring the amplitude, overwrite signal amplitude, noise level amplitude and pulse width of the electrical signals and for producing, as an output from the system, digital representations thereof in the form of a multibit digital output signal capable of being processed by a computer or other digital processing circuits to produce output information therefrom relating to the amplitude, read-over-write ratio, signal-to-noise ratio and average pulse widths of the electrical signals.
2. Description of the Prior Art
It is known in the prior art to utilize apparatus, equipment and systems for measuring selected parameters of electrical signals derived from magnetic recording. One such known system is offered for sale and sold by Applied Magnetics Corporation and is known as the Model HTU-280 Magnetic Head Tester for Thin-Film Heads. In the Model HTU-280 thin film head tester, the apparatus includes means for measuring the peak average amplitude and the pulse widths of the electrical signals utilizing analog signal processing circuitry. The processing rates are at relatively low frequencies in the order of 100 to 200 hertz. The peak average amplitude detectors are responsive to the electrical signals which have a relatively high frequency relative to the processing frequency of the measuring circuitry and the detectors measure changes or variations in the envelope of the electrical signals rather than the individual amplitude or pulse width of a signal.
Circuits for measuring the pulse width of electrical signals are well known in the art. Typical of such pulse width measuring systems is a system offered for sale by Hewlett-Packard Corporation, identified as Model 300 Pulse Counter. The Model 300 Pulse Counter measures pulse widths utilizing means for converting the pulse into a square wave at a fast clocking rate and counting the pulses with a high-speed counter.
It is also known in the art to use systems and apparatus for developing the "read-over-write" characteristics of a magnetic head in recording and reproducing prerecorded magnetic data from a magnetic medium. The read-over-write measuring technique involves writing a signal onto the magnetic disc with a magnetic head at a predetermined frequency and then erasing the recorded frequency by recording a second signal at a different higher frequency on the same portion of the magnetic disc. The read-over-write measurement comprises a ratio of the amplitude of the previously recorded signal at a first frequency to the amplitude of the presently recorded signal at a second frequency, which frequency is preferably at a higher frequency. The ratio between the two signals is referred to as the read-over-write ratio. Typical frequencies utilized in the IBM Model 3370 technology for making the read-over-write measurement are a low-frequency signal, in the order of 1.8 megahertz, as the first signal and a high-frequency signal, in the order of 4.8 megahertz, as the second signal. In the IBM Model 3380 type system, the low-frequency signals are in the order of 3.0 megahertz, and the high-frequency signals are in the order of 8.0 megahertz.
Another known device for measuring the frequency and other characteristics of electrical signals derived from magnetic recording is Model SLM telemetering apparatus which is offered for sale and sold by Hewlett-Packard Corporation. The Model SLM telemetering apparatus operates at a fixed frequency and does not track minute changes in frequency versus variation of disc rotation. The Model SLM telemetering apparatus detects the signal-to-noise ratio and uses a true RMS signal for purposes of detecting the signal-to-noise ratio.
Typically, most prior art measuring devices include analog peak detectors and analog-to-digital converters in order to generate a digital representation of the amplitude for input to a computer or other digital device.
It is also known in the art to utilize a number of recording schemes or formats for recording electrical signals. Typically, the recorded signal outputs from a magnetic head in a magnetic recording and reproducing apparatus have a frequency in the order of 1 to 10 megahertz or higher. The electrical signals derived from the magnetic recording may be modulated by any conventionally used modulation codes, such as multiple frequency modulation ("MFM"), biphase coding, limited run length codes, or any other coding system such as phase modulation or the like. The system and apparatus of the present invention are able to measure the selected parameters of the electrical signals independent of the recording format.